Continuum star Lexa Doig has been cast as Talia al Ghul for CW's Arrow, according to Deadline. Described as "an elite warrior who doesn't pick sides but rather creates her own," Doig's said to be appearing in several episodes.
Talia's father Ra's al Ghul (played by Matt Nable) and half-sister Nyssa (Katrina Law) have been major components of Arrow, and the Talia character actually had a cameo as a child in DC's Legends of Tomorrow, played by Milli Wilkinson.
Doig is a veteran of several sci-fi shows, having appeared in Continuum, Stargate SG-1, Andromeda, The 4400, and Supernatural. She has several comic book-inspired roles on her resume as well, voicing Dale Arden in the 1990s Flash Gordon cartoon, and appeared as Dr. Christina Lamell in Smallville.